Structural change + API change

Changes include:
* plugin API change (is now unified for all plugins),
* external bitstamp dependency removed,
* public `sell` method added,
* now tests cover plugin's public API,
* [travis](https://travis-ci.org) support added,
* _plugin-developers documentation (and generator) on the way._

Breaks:
* balance returned,
* `.purchase()` now takes `options` object as a second argument, 
* initialization process (**requires server update**).
Document why we need this.